[48.10] Diffuse Ionized Gas in Sculptor Group Galaxies

C. Hoopes, R. A. M. Walterbos, B. Greenawalt (NMSU)

Diffuse Ionized Gas (DIG) is an important component of the
interstellar medium, containing most of the mass of ionized gas.
The volume filling factor of DIG for the Milky Way is about 0.2
in the disk. This diffuse gas (n_e\approx0.2 cm^-3) is
detected as faint emission in H\alpha and forbidden lines
outside of HII regions. Previous observations have found that the
[SII](6717+6731ÅH\alpha intensity ratio of the DIG is higher than
that of HII regions, making it possible to distinguish the two types of
emission through narrow-band imaging in these lines. Although edge-on
spirals are being extensively studied for the presence of DIG in the
halo, few studies have been done of DIG in face-on spiral galaxy disks.
We present the results of a study of three Sculptor Group galaxies:
NGC55 (an edge-on SBm galaxy), NGC253 (an inclined starburst Sc
galaxy), and NGC300 (a face-on Sd galaxy). Narrow-band H\alpha
and [SII] images were analyzed to characterize the extent and
nature of the DIG. The images show the presence of DIG in all three
galaxies, and the [SII]/H\alpha ratio images confirm the enhancement
of [SII] emission in the diffuse gas. We find that the distribution of
DIG in the disk is strongly correlated with that of HII
regions in the spiral arms. We measure the H\alpha luminosity
for each galaxy, and find
that in all three the DIG contributes 30-40% of the total. This
result is consistent with studies of DIG in other nearby galaxies, and
is surprising given the markedly different nature of these
galaxies. We also investigate the outflow cone from the starburst
nucleus of NGC253, and find that the [SII]/(H\alpha+[NII]) ratio is
intermediate between that of HII regions and the DIG, possibly indicating a
mixture of photoionization and shock ionization in the outflow.